A phrase used for certain Harry Potter couples and pairings that you adore so much to the point where saying 'I ship it' becomes an understatement, hence, the use of 'I Durmstrang ship it' becomes more appropriate.
Because the Durmstrang ship is a magical ship and can travel underwater, it implies that a particular pairing is magical, can survive everywhere (on land and underwater too), and you willalways ship it, no matter what.